We propose to analyze the cytological and molecular mechanisms of the eukaryotic cell cycle by utilizing various mutant strains and developmental stages of Saccharomyces cerevisiae. The spindle pole bodies play a central role here in controlling cell division and coordinating various cell cycle events. Changes in the structure and functional properties of these bodies and their attached microtubules during nuclear division and karyokinesis will be analyzed in cell division cycle mutants. Polymerization of purified tubulin under control of isolated spindle pole bodies will be assayed in vitro. In addition, we shall analyze the control of expression by particular cdc mutants by using the cloned genes to probe the production of RNA in the cell cycle.